1. Technical Field
This invention relates generally to metal gaskets. More particularly, it relates to embossed metal gaskets made from high temperature alloys. Even more particularly, to embossed metal gaskets made from high temperature iron-nickel-chromium alloys that are adapted to operate at temperatures greater than 1100° F.
2. Related Art
It is well known to use embossed metal gasket sheets for high temperature clamped joints of internal combustion engines, including Diesel engines, that require a fluid seal, such as head and exhaust gaskets. These embossments can be formed in many shapes and sizes, including full and half embossments, depending on the fluid channel to be sealed, but in the case of full embossments will frequently comprise a substantially circular arc that is a function of the thickness, T, of the sheet with a width on the order of about 10-20 T and a height of about 0.5-2.5 T.
Many metal sheet materials, including numerous types of steels and stainless steels have been used to make embossed metal gaskets. For example, type 301 stainless steel fully hardened by cold reduction (301 FH SS) has been successfully used in many gasket applications with operating temperatures up to about 800-900° F. However, at temperatures above about 1000° F. this material is susceptible to creep, stress relaxation and other high temperature phenomena such that it loses a significant portion of its strength causing the embossments take a heat set and fail to maintain, or recover once unclamped, a sufficient amount of the original embossed height to provide an operable seal. Thus, 301 FH SS is not a desirable gasket material for these applications either because it loses the ability to effectively and operably seal many clamped joints altogether, or because the use of this material would require too many gasket layers to economically or reliably provide the seal height necessary to operably seal the joint.
As described in published patent application EP1429057 A1, Type 316Ti stainless steel alloy has somewhat improved high temperature strength and other properties, including height recovery properties, as compared to type 301 FH SS, which make it suitable for use in certain high temperature embossed metal gasket applications. This material performs well at temperatures somewhat above 800-900° F., but also loses sufficient strength at temperatures above about 1000° F., where the embossments also take a heat set and fail to recover a sufficient amount of their original embossed height to provide an operable seal as described above.
As internal combustion engines continue to employ new technologies to improve the combustion process and achieve exhaust emission reductions, increased fuel efficiency and the like, the operating temperatures to which the clamped joints and embossed metal gaskets used in these engines are exposed continue to increase, making materials that once performed well in gasket applications, such as various types of stainless steels, including 301 FH SS and 316 Ti, either undesirable or unsuitable for use in certain applications with respect to economic, performance or other considerations. The recovery performance of various stainless steel alloys currently used as gasket materials is shown in FIG. 1 to illustrate this point. As the recovered height performance decreases, particularly at temperatures exceeding 1000° F., the number of gasket layers required to operably seal a joint increases.
For example, the trend in the design of at least some modern Diesel engines for marine applications has been to insulate the exhaust system (e.g., the exhaust manifold, down pipe and/or muffler) in order to eliminate water jacket cooling, thereby increasing the operating temperature of the joint between the exhaust manifold and engine block, and the associated metal gasket, to temperatures above those which can be effectively sealed by 301 FH or 316 Ti stainless steels.
In another example, other Diesel engines are designed to recirculate a portion of the exhaust gas stream back into the combustion chamber in an effort to reduce emissions, which adds additional hot components and joints that must be sealed for the engine and engine control systems to operate properly. In many applications, temperatures in the vicinity of the exhaust manifold gasket exceed 1100° F. and can reach temperatures between 1400 to 1600° F. or more, making the traditional 301 FH SS and 316 Ti alloys unsuitable as gasket material.
One material that has been used successfully for gasket applications that exceed 1100° F. is Inconel® 718. In these applications, gasket sheets are formed to shape and embossed in the annealed (i.e., solution heat treated) state to provide sufficient ductility to perform these processes without cracking the material. They are then given a precipitation hardening heat treatment following embossment at temperatures of between about 1200-1350° F. to harden and strengthen the gaskets to provide sufficient strength to the gasket sheet and embossments for operation at temperatures above 1100° F. While suitable from a performance standpoint, this material is considerably more expensive than the 301 FH SS and also requires post-embossment heat treating which adds to cost and complexity to the manufacture of these gaskets.
Therefore, there remains a need for improved embossed gaskets for use at operating temperatures above those for which 301 FH SS and 316Ti may be used, as well as materials and methods of their manufacture.